The active contractile components of skeletal muscle have and continue to be studied extensively. In contrast, passive connective tissue components have been neglected. This neglect is puzzling as disorders such as muscular dystrophy, myositis, ischemic muscular necrosis and others are associated with intramuscular connective tissue changes. Further, neural and vascular channels to the active components pass within the surrounding and very plastic connective tissue network. the aims of this project are 1) to determine the changes in the quantity and composition of intramuscular connective tissue when muscles are stressed by over-use; 2) to correlate the changes in muscular connective tissue with twitch tension, tetanic tension and fatigability; 3) to determine if changes are age and load dependent; and 4) to establish a baseline for future studies involving pathological neuromuscular disorders. Juvenile (3-4 weeks) and adult (12-13 weeks) syrian hamsters will be subdivided randomly into a control group and two experimental groups. In experimental group I, muscle over-use will be accomplished by unilateral detachment of a synergist muscle in the leg. In experimental group II, muscle over-use in the leg muscles will be accomplished by applying subcutaneous loads in the lumbosacral region that are carried by the animals for 6 weeks. For all groups, (1) the percent composition of collagen, elastic and reticular fibers will be determined by serial histological procedures; (2) the quantity of connective tissue will be measured using computer digitizing techniques and (3) the physiological properties of the muscles determined and correlated with morphological changes. It is hypothesized that (1) tensile stresses in over-used muscles increase the quantity of intramuscular collagen, but not elastin or reticulin, increasing muscle stiffness but not maximum tetanic tension, (2) increased stiffness produces increased fatigability due to capillary compression, and (3) the magnitude of connective tissue changes are age dependent.